Phase change materials (PCMs) are used to store and release thermal energies through phase transformations—most commonly melting and crystallization. The energy which is stored and released in these transitions is known as latent heat, and latent heat storage is among the most effective techniques for storing thermal energy.
In commercial and industrial settings, several distinct classes of PCMs are used for heat storage. Different temperature ranges between the classes of PCMs generally dictates which type of PCM will be suitable for a specific application. PCMs are most often divided into classifications based on their chemical composition. Inorganic PCMs such as hydrated and molten salts are commonly used for high temperature applications, while organic PCMs such as fatty acids and paraffin waxes are used at lower temperatures. Paraffin waxes are one of the most widely used PCMs. The melting point of a paraffin wax is dependent on the number of carbon atoms in the paraffin and generally increases with the average carbon number.
Due to the environmental implications and increasing economic costs associated with limited oil reserves and corresponding increasing costs associated with petrochemicals, renewable alternatives for paraffin wax are attracting significant attention. Analysis of the constituents of paraffin waxes in candles shows that they also contain hazardous materials such as toluene, trichloroethylene, and a variety of alkanes and alkenes, many of which have unknown safety implications. Due to these toxicity issues and the non-renewability of the feedstock, interest and demand for other types renewable PCMs has been growing. Fatty acid derived materials are suitable candidates for the substitution of paraffin wax in many applications—they may be non-toxic, are renewable, and are less corrosive than inorganic PCMs. However, the fatty acid derived PCMs produced to date have melting temperature and latent heat values lower than those of paraffin wax, resulting in lower energy storage densities. For example, triglycerides have been transesterified with a short chain alcohol to create fatty acid alkyl esters for use as PCMs. In such applications, modifications of the feedstock's oil or hydrogenation of the double bond was reported to provide blends of fatty esters with varying melting points and latent heat values between 110-140 J/g. Although the PCMs that were produced are derived from renewable feedstocks, the melting points and latent heat values compare unfavorably with paraffins. Thus, use of these fatty acid alkyl ester PCMs would require a significantly larger PCM mass to store the same amount of thermal energy as paraffin waxes, which have been reported to have latent heats between about 146-210 J/g.